Dental imaging with photon-counting detector

ABSTRACT

An extra-oral dental imaging apparatus for obtaining an image from a patient has a radiation source; a first digital imaging sensor that provides, for each of a plurality of image pixels, at least a first digital value according to a count of received photons that exceed at least a first energy threshold; a mount that supports the radiation source and the first digital imaging sensor on opposite sides of the patient&#39;s head; a computer in signal communication with the digital imaging sensor for acquiring a first two-dimensional image from the first digital imaging sensor; and a second digital imaging sensor that is alternately switched into place by the mount and that provides image data according to received radiation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of earlier filed application U.S.Ser. No. 14/366,696 , filed on Jun. 19, 2014, entitled “DENTAL IMAGINGWITH PHOTON-COUNTING DETECTOR”, in the names of Inglese et al., which isitself a is a 371 national stage application of earlier filedinternational application Serial No. PCT/US2012/043510, filed on2012-Jun.-21, entitled “DENTAL IMAGING WITH PHOTON-COUNTING DETECTOR”,in the names of Inglese et al., which application itself claims thebenefit of earlier filed international application Serial No.PCT/US2011/066432, filed on 2011-Dec.-21, entitled “DIGITAL DETECTOR”,in the names of Inglese et al., all of which are incorporated herein intheir entirety.

FIELD OF THE INVENTION

The invention relates generally to the field of extra oral dentalimaging and more particularly to apparatus and methods for obtainingvolumetric images from the head of a patient.

BACKGROUND OF THE INVENTION

Radiological imaging is recognized to have significant value for thedental practitioner, helping to identify various problems and tovalidate other measurements and observations related to the patient'steeth and supporting structures. Among x-ray systems with particularpromise for improving dental care is the extra-oral imaging apparatusthat is capable of obtaining one or more radiographic images in a seriesand, where multiple images of the patient are acquired at differentangles, combining these images to obtain a contiguous panoramicradiograph of the patient showing the entire dentition of the jaw, atomographic image that contains more depth detail, or a computerizedtomography (CT) volume image. To obtain images of any of these types, aradiation source and an imaging detector, maintained at a fixed distancefrom each other, synchronously revolve about the patient over a range ofangles, taking a series of images by directing and detecting radiationthat is directed through the patient at different angles of revolution.

Combination systems that provide both CT and panoramic x-ray imaging caninclude an X-ray source, an X-ray detector for detecting X-rays havingpassed through the subject, and supporting means for supporting theX-ray source and the X-ray detector so that they are spatially opposedto each other across the subject; and mode switching means for switchingbetween a CT mode and a panorama mode. To detect X-rays, only one largearea X-ray detector is used. The X-ray imaging apparatus can obtain bothtypes of images by switching modes during the imaging session. However,the proposed imaging apparatus performs both CT and panoramic imagingusing only one detector. This requires an expensive detector capable ofcarrying out both imaging functions in a satisfactory manner.

A combination imaging system can provide both CT and panoramic imagingusing two separate sensors or detectors. By way of example, FIG. 1 inthe present application shows a combined panoramic and CT imagingapparatus 40. A telescopic column 18 is adjustable for height of thesubject. The patient 12 or other subject, shown in dotted outline, ispositioned between an x-ray source 10 and an x-ray imaging sensor panel20. X-ray imaging sensor panel 20 rotates on a rotatable mount 30 inorder to position either a CT or a panoramic sensor 21 for obtaining theexposure. For CT imaging, CT sensor 21 is positioned behind the subject,relative to x-ray source 10. The operator rotates CT sensor 21 into thisposition as part of imaging setup. Similarly, the operator rotatespanoramic sensor 21 into position behind the subject as part of thesetup for a panoramic imaging session.

Another recent imaging system combines CT, panoramic, and cephalometricimaging from a single apparatus. For example, commonly assigned U.S.Patent Application Publication No. 2012/0039436 entitled “COMBINEDPANORAMIC AND COMPUTED TOMOGRAPHY APPARATUS” to Bothorel et al.describes such a system.

A computerized tomography (CT) imaging apparatus operates by acquiringmultiple 2D images with a rotating imaging ensemble or gantry that hasan x-ray source and, corresponding to (e.g., opposite) the x-ray source,an imaging sensor having a selectable spatial relationship (e.g.,rotating about a fixed axis) relative to the patient. CT imaging allowsthe reconstruction of 3D or volume images of anatomical structures ofthe patient and is acknowledged to be of particular value for obtaininguseful information for assisting diagnosis and treatment.

Conventional digital radiography detectors have some limitations relatedto how attenuation of radiation energy at a single exposure isinterpreted. For example, it can be very difficult, from a singleexposure, to distinguish whether an imaged object has a given thicknessor a given attenuation coefficient. To resolve this ambiguity, somesystems provide separate, sequential low-energy and higher energyexposures and use the resulting difference in image information todistinguish between types of materials. However, in order to providethis information, this type of imaging requires that the patient besubjected to additional radiation for the second exposure. This problemcan be compounded for CT imaging, in which multiple images are obtained,one from each of a number of angles of revolution about the patient.

Conventional CT imaging provides useful information that aids indiagnosis and treatment, but is constrained by limitations of theimaging sensor apparatus itself, and there are concerns over exposurelevels needed for obtaining the desired image quality. There is room forimprovement in system performance and in providing types of imaging thataddress practitioner interests with respect to a particular patient.Improvements are also needed for more accurate equipment positioningrelative to the patient as well as for overall patient comfort.Additional improvements in image acquisition sequences and processingare also desired.

SUMMARY OF THE INVENTION

Accordingly, it is an aspect of this application to address in whole orin part, at least the foregoing and other deficiencies in the relatedart.

It is another aspect of this application to provide in whole or in part,at least the advantages described herein.

Embodiments of the present invention address the need for advancing theCT imaging art, particularly for imaging of the head. Embodiments of thepresent invention adapt photon-counting and related imaging solutions tothe problem of CT imaging for dental, ENT, and related applications.Using embodiments of the present invention, a medical practitioner canobtain useful images for patient treatment, taking advantage of reducedexposure levels and other advantages that photon-counting solutionsprovide.

These objects are given only by way of illustrative example, and suchobjects may be exemplary of one or more embodiments of the invention.Other desirable objectives and advantages inherently achieved by thedisclosed invention may occur or become apparent to those skilled in theart. The invention is defined by the appended claims.

According to one aspect of the invention, there is provided anextra-oral dental imaging apparatus for obtaining an image from apatient, the apparatus comprising: a radiation source; a first digitalimaging sensor that provides, for each of a plurality of image pixels,at least a first digital value according to a count of received photonsthat exceed at least a first energy threshold; a mount that supports theradiation source and the first digital imaging sensor on opposite sidesof the patient's head; a computer in signal communication with thedigital imaging sensor for acquiring a first two-dimensional image fromthe first digital imaging sensor; and a second digital imaging sensorthat is alternately used (e.g., switched into place by the mount) andthat provides image data according to received radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of the embodiments of the invention, as illustrated in theaccompanying drawings.

The elements of the drawings are not necessarily to scale relative toeach other.

FIG. 1 shows a conventional CT imaging apparatus for dental orear-nose-throat (ENT) imaging.

FIG. 2 shows an imaging apparatus that provides both CT and panoramicx-ray imaging and adds cephalometric imaging capability.

FIG. 3 shows an imaging apparatus according to an embodiment of thepresent invention.

FIG. 4A is a schematic view that shows a digital detector using ascintillator in conventional digital radiographic imaging.

FIG. 4B is a schematic view that shows a digital detector using ascintillator with a fiber optic array in conventional digitalradiographic imaging.

FIG. 4C is a schematic view that shows a digital detector using athicker scintillator with a fiber optic array in conventional digitalradiographic imaging.

FIG. 4D is a schematic view that shows a digital detector using astructured scintillator with a fiber optic array in conventional digitalradiographic imaging.

FIG. 5 is a schematic view that shows a digital detector using a photoncounting for digital radiographic imaging.

FIG. 6 is a schematic diagram that shows the image processing chain foreach pixel of the digital detector when using photon counting.

FIG. 7 is a schematic diagram that shows the image processing chain foreach pixel of the digital detector using multiple thresholds when usingphoton counting.

FIG. 8A is a graph that shows linear attenuation characteristics atdifferent energy levels for two exemplary metallic materials.

FIG. 8B is a graph that shows the linear absorption coefficient fordifferent types of bone tissue.

FIG. 9A shows a schematic view of source-to-detector distances thatapply for each type of imaging that is performed by apparatus of thepresent invention.

FIG. 9B shows a schematic view with variable source-to-detectordistances for different imaging modes.

FIG. 10A shows an embodiment of a three-position detector positioningapparatus.

FIG. 10B shows the use of the detector positioning apparatus of FIG. 10Afor CT imaging.

FIG. 10C shows the use of the detector positioning apparatus of FIG. 10Afor panoramic imaging.

FIG. 10D shows the use of the detector positioning apparatus of FIG. 10Afor cephalometric imaging.

FIG. 11A shows an alternate embodiment of a three-position detectorpositioning apparatus.

FIG. 11B shows the use of the detector positioning apparatus of FIG. 11Afor CT imaging.

FIG. 11C shows the use of the detector positioning apparatus of FIG. 11Afor panoramic imaging.

FIG. 11D shows the use of the detector positioning apparatus of FIG. 11Afor cephalometric imaging.

FIG. 12A shows another alternate embodiment of a three-position detectorpositioning apparatus.

FIG. 12B shows the use of the detector positioning apparatus of FIG. 12Afor CT imaging.

FIG. 12C shows the use of the detector positioning apparatus of FIG. 12Afor panoramic imaging.

FIG. 12D shows the use of the detector positioning apparatus of FIG. 12Afor cephalometric imaging.

FIG. 12E shows a scan pattern used for partial CT imaging according toan embodiment of the present invention.

FIG. 13 is a schematic diagram showing an imaging apparatus for imagingportions of the patient's head using photon counting.

FIG. 14 is a schematic diagram that shows a portion of a helical scanfor the digital sensor and radiation source.

FIGS. 15A and 15B show the imaging apparatus that provides a helicalscan by changing the elevation of a support arm during revolution aboutthe patient.

FIGS. 16A and 16B show the imaging apparatus that provides a helicalscan by changing the elevation of the digital sensor and radiationsource during revolution about the patient.

FIGS. 17A and 17B show the imaging apparatus that provides a helicalscan by changing the elevation of the patient's head relative to thedigital sensor and radiation source during revolution about the patient.

FIG. 18 is a logic flow diagram showing steps for image acquisitionaccording to an embodiment of the present invention.

FIG. 19 is a schematic view showing components of a patient supportapparatus.

FIG. 20A shows features of a head support for patient imaging.

FIG. 20B shows alternate features of a head support for patient imaging.

FIG. 21 shows use of a mask used for head and chin support during avolume imaging session.

FIG. 22 shows components of a mask used for head and chin support duringa volume imaging session.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following is a detailed description of the exemplary embodiments ofthe invention, reference being made to the drawings in which the samereference numerals identify the same elements of structure in each ofthe several figures.

According to certain exemplary embodiments, there is provided aradiographic imaging apparatus and/or methods (e.g., a dental,extra-oral dental) for obtaining an image based on received radiation(e.g., from a patient) that can provide the capability to obtain theimage using at least one imaging modality (e.g., panoramic imaging,cephalometric imaging and CT imaging) and configured with at least onedetector (e.g., sensor) of the photon counting type. When multipleimaging modalities are present in a single radiographic imagingapparatus, either additional detectors of the photon counting type orother types (e.g., CMOS-based, CCD-based, flat panel area detectors,sensors that use amorphous or polycrystalline semiconductor materialssuch as a-Si) can be used and/or at least one detector can be sharedamong at least two imaging modalities.

According to certain exemplary embodiments, the photon counting sensorcan equip extraoral dental imaging device. In at least one embodiment,extra oral dental imaging devices can include at least one of aComputerized Tomography sensor, a panoramic sensor or a cephalometricsensor of the photon counting type. The cephalometric image acquisitionmethod can be one shot or slot-scan.

For example, an extra oral dental imaging device can comprise twosensors, namely either a combination of a panoramic sensor and a CTsensor, a combination of a panoramic sensor and a cephalometric sensoror a combination of panoramic sensor and a CT sensor where at least oneof the two sensors is the photon counting type. A device in which onesingle sensor is used for the two sensing functionalities can also becontemplated. In that case, the sensor is depluged from a first positioncorresponding to a first imaging functionality and is plugged to asecond position corresponding to a second imaging functionality.

Further, an extra oral imaging device comprising the threefunctionalities panoramic imaging, cephalometric imaging and CT imagingcan also be contemplated, where at least one of the three imagingfunctionalities uses photon counting processes. In at least oneembodiment, less than three sensors can be used for the three imagingfunctionalities. For example, at least one of the less than threesensors can be unplugged from a first position and plugged in a secondposition.

In the context of the present disclosure, the terms “pixel” and “voxel”may be used interchangeably to describe an individual digital image dataelement, that is, a single value representing a measured image signalintensity. Conventionally an individual digital image data element isreferred to as a voxel for 3-dimensional volume images and a pixel for2-dimensional images. Volume images, such as those from CT or CBCTapparatus, are formed by obtaining multiple 2-D images of pixels, takenat different relative angles, then combining the image data to formcorresponding 3-D voxels. For the purposes of the description herein,the terms voxel and pixel can generally be considered equivalent,describing an image elemental datum that is capable of having a range ofnumerical values. Voxels and pixels have attributes of both spatiallocation and image data code value.

In the context of the present disclosure, the term “code value” refersto the value that is associated with each volume image data element orvoxel in the reconstructed 3-D volume image. The code values for CTimages are often, but not always, expressed in Hounsfield units (HU).

In the context of the present invention, the terms “digital sensor” or“sensor panel” and “digital detector” are considered to be equivalent.These describe the panel that obtains image data in a digitalradiography system. The term “revolve” has its conventional meaning, tomove in a curved path (e.g., 3D) or orbit around a center point (e.g.,fixed or movable).

In the context of the present disclosure, the terms “operator”, and“user” are considered to be equivalent and refer to the operatingpractitioner, technician, or other person who views and manipulates anx-ray image or a volume image (e.g., formed from a combination ofmultiple x-ray images) at an image acquisition apparatus (e.g., on adisplay monitor). A “technician instruction” or “operator command” canbe obtained from explicit commands entered by the user or may beimplicitly obtained or derived based on some other user action, such asmaking a collimator setting, for example. With respect to entries on anoperator interface, such as an interface using a display monitor andkeyboard, for example, the terms “command” and “instruction” may be usedinterchangeably to refer to an operator entry.

In the context of the present disclosure, the terms “viewer” can referto the viewing practitioner (e.g., radiologist), technician, or otherperson who views and manipulates an x-ray image or a volume image thatis formed from a combination of multiple x-ray images, on a displaymonitor away form the image acquisition apparatus. A “viewerinstruction” can be obtained from explicit commands entered by theviewer or may be implicitly obtained or derived based on some otheraction.

In order to more fully understand aspects of the present invention, itis instructive to consider different approaches used for CT imaging inconventional practice and to compare these with aspects of CT and othervolume imaging modes according to embodiments of the present invention.By way of example, FIG. 1 shows an embodiment of a conventional CTimaging apparatus 40 for dental imaging. A column 18 is adjustable forheight of the subject. The patient 12 or other subject, shown in dottedoutline, is positioned between an x-ray source 10 and an x-ray imagingsensor panel 20, also termed an imaging detector. X-ray imaging sensorpanel 20 rotates on a rotatable mount 30 in order to position a CTsensor 21 for obtaining the exposure. CT sensor 21 is positioned behindthe subject, relative to x-ray source 10. The operator rotates CT sensor21 into this position as part of imaging setup. With rotation of mount30, sensor 21 and source 10 revolve about the head of the patient,typically for some portion of a full revolution.

Other imaging system solutions provide additional types or modes ofimaging in addition to CT imaging and thus enable switching betweenvarious imaging modes. FIG. 2 shows an embodiment of an imagingapparatus providing combined panoramic, CT, and cephalometric imaging.An imaging apparatus 50 has similar radiation source and sensorcomponents to the earlier system of FIG. 1 , provided on rotatable mount30. In addition, a cephalometric imaging system 31 is mounted on aseparate arm 32.

Referring to the perspective view of FIG. 3 , a combined imagingapparatus 100 for panoramic, computed tomography and cephalometricimaging has a base 195, a support pole 190, and an elevation member 170mounted on support pole 190. Elevation member 170 adjusts over a rangeof vertical positions to adapt for patient height. A rotary armsupporting member 150 extends from an upper portion of elevation member170. A rotary arm 140 is supported by rotary arm supporting member 150and provides, at one end, an x-ray source 110 that is energizable toprovide exposure radiation along an exposure path and, at the other end,an x-ray detector apparatus 120. X-ray source 110 is in a fixed positionrelative to rotary arm 140 in one embodiment. In an alternateembodiment, x-ray source 110 can be separately mounted and moved towardor away from x-ray detector apparatus 120, in the x-direction as notedin FIG. 3 . Not shown in FIG. 3 , but required for an imaging apparatusof this type, is the needed support apparatus for providing power, dataconnection, and other functions.

Windowing can alternately be used for obtaining image content that canbe used for reconstructing the volume image. According to an embodimentof the present invention, a movable slit translates along the surface ofthe imaging sensor to provide a scan of the image content. This slit canbe provided by the collimator, for example, or by some alternate deviceor may be digitally simulated. Alternately, movement of the imagingsensor itself is used to determine the imaged area. In certain exemplaryembodiments, windowing can be provided by control of at least oneselected portion (e.g., square, slit, prescribed area, prescribedsubset) of the sensor (e.g., pixels) that are configured to be read outto generate an image (e.g. based on radiation received). The selectedportion (e.g., the window) can correspond to a capability of an imagingsystem or an examination type and can be implemented by hardware,software or a combination thereof.

Other types of image acquisition and processing can alternately beperformed using the same basic system, including 2-D image acquisition,partial CT imaging, transverse imaging (in which the layer of interestis orthogonal to the panoramic layer), SPECT (Single-Photon EmissionComputerized Tomography) scanning, linear tomographic imaging, andothers.

Photon-Counting Detector Operation and Data Acquisition

For each of the embodiments shown in FIGS. 1-3 , a sensor panel is usedas the digital detector for receiving the exposure and generating theimage data. For embodiments of the present invention, one or more of thesensor panels that is used for acquiring data is a photon-counting imagedetector.

FIGS. 4A through 4D schematically illustrate different approaches toradiologic imaging that can be employed for CT systems. FIG. 4A showselements of an x-ray imaging sensor 50 that uses an indirect imagingmethod for generating image data in response to radiation through apatient or other subject. In this model, x-ray photons are incident onan x-ray converting element 52 that converts the energy from ionizingx-ray radiation to visible light or other light energy. X-ray convertingelement 52 is commonly referred to as a scintillator. An energydetecting element 54, mounted on a support structure 56, then detectsthe converted energy, such as using an array of photocells. Thephotocells can be light-sensitive CMOS (Complementary Metal-OxideSemiconductor) components formed in an array as a semiconductor chip andproviding a signal corresponding to each detected image pixel.

Scatter, resulting in cross-talk between pixels and consequent loss ofsome amount of resolution, is one acknowledged problem with the basicapproach shown in FIG. 4A. The modification of FIG. 4B addresses thisproblem by adding a fiber-optic array 60 between the scintillator orx-ray converting element 52 and energy detecting elements 54. FIG. 4Cshows another modification that can help to improve sensitivity toradiation, enlarging the width of the scintillator or x-ray convertingelement 52; however, this solution can result in some loss of sharpnessin the obtained image.

FIG. 4D shows the use of a structured scintillator serving as x-rayconverting element 52. The structured scintillator can use a materialsuch as cesium iodide (CsI), although this material is structurallyfragile and has some limitations with respect to image quality. Thismodified scintillator type can be used in addition to fiber-optic array60 as shown in FIG. 4D for some improvement in performance.

The conventional model shown in FIG. 4A and improvements outlined withrespect to FIGS. 4B, 4C, and 4D provide a reasonable level of imagingperformance for CT imaging applications. However, even with the addedcost and complexity of the additional components and features used, onlyincremental improvements in image quality and overall performance areachieved.

An alternative approach to image capture using a direct imaging methodis shown in FIG. 5 . An imaging sensor 70 using direct detection has adirect detection element 72, such as a semiconductor or other sensitivematerial that converts incident x-ray photons to an electron flow. Theexcited electrons are then accelerated by an electrical field F and aresensed by an electron-sensitive CMOS array that acts as energy detectingelement 54. Advantageously, with direct detection imaging sensor 70,each incoming x-ray photon is much more likely to be detected than withindirect imaging devices. This increases the DQE (detective quantumefficiency), a performance metric for an imaging detector. Reducedscatter, a result of the electric field that guides electron chargetoward the CMOS array elements, makes this approach more efficient,improves resolution, and provides a more favorable signal-to-noise (S/N)ratio. As a result, lower levels of ionizing radiation can be used forobtaining an image with direct detection imaging sensor 70. The neededthreshold values are within or below the range needed with the moreconventional indirect devices described with reference to FIGS. 4A-4D.Direct-detection semiconductors used for direct detection element 72 caninclude polycrystalline or monocrystalline materials. Monocrystallinematerials are advantaged over polycrystalline for ease of fabricationand handling; however, there are size constraints to detectors formedfrom monocrystalline materials. Polycrystalline materials are moredifficult to fabricate and handle, but are capable of providing largerdetectors. Candidate materials for this purpose include cadmiumtelluride (CdTe or CadTel), lead iodide (PbI₂), lead oxide (PbO), andmercuric iodide (HgI₂), and types of poly crystal, amorphous Selenium(aSe), and other materials.

Another distinction is made between how x-ray detectors record andreport the received energy. Integrating x-ray sensors are spatiallydigitized and provide an analog output that represents the accumulatedcharge received for each pixel during the exposure. High noise levelscan be a problem with integrating sensors. Another approach is commonlytermed “photon-counting”. In this alternative method, each incomingphoton generates a charge, and each of these events is reported orcounted. The actual count of photons, or a value that is computedaccording to the count, is provided as the image data for each pixel.Advantageously, photon counting has high immunity to noise, providedthat pulse strength exceeds background noise levels. FIG. 6 shows thephoton-counting sequence in schematic form. An incoming photon generatesa pulse 80 at a given energy level. The pulse 80 energy is comparedagainst a threshold value at a comparator 82 and shaped in a pulseshaper 84 to form a shaped pulse 88. A counter 86 then records the pulseevent and provides a digital output, a pulse count value 90. A separatepulse count value 90 is obtained for each pixel element in imagingsensor 70. The threshold value can be adjustable or selectable from arange of values, depending on the photon energies of interest.

A further advantage of pulse counting relates to its capability to countpulses 80 at multiple threshold values. Referring to the schematicdiagram of FIG. 7 , two comparators 82 a and 82 b are shown formeasuring pulse energy. In this particular configuration, a comparator82 a, a pulse shaper 84 a, and a counter 86 a provide a count 90 a valuefor all pulses above a first threshold; similarly, a comparator 82 b, apulse shaper 84 b, and a counter 86 b account for only pulses above ahigher, second threshold and provide a count 90 b accordingly. Simplesubtraction then identifies the different power levels achieved for eachpulse. It can be appreciated that more than two threshold levels can bemeasured, using a corresponding arrangement of comparator circuitry thatfollows the model that is shown in FIG. 7 , allowing pulse counts at anyof a number of threshold values. In addition, thresholds can beselectable, such as adjustable to adjust the response of imaging sensor70 to various photon energy levels. Thus, for example, an operator canuse a set of preset thresholds for differentiating softer from densertissue in the image that is finally generated.

In addition to setting minimum thresholds, embodiments of the presentinvention also provide the option of using upper or maximum thresholdsfor photon energy. This capability can be used for a number offunctions, including reducing the effects of radiation on semiconductorcomponents and reducing generation of excessive noise signals.

The capability to count photons at different energy thresholds, asdescribed with reference to FIG. 7 , allows the sensor to differentiatebetween energy levels obtained from irradiating the subject and providesadded dimension to the image data that is provided as a result of eachexposure. This capability, described as multi-spectral or “color” x-rayimaging, enables information to be obtained about the materialcomposition of a subject pixel. As shown for typical metals in thesimplified graph of FIG. 8A, two materials A and B have differentcoefficients of attenuation β that vary with the level of radiationenergy, shown as exposure E. At a given exposure, material A attenuatesa photon with an energy that corresponds to material A, as shown atvalue A11. Similarly, radiation impinging on material B attenuates aphoton with an energy that corresponds to material B, as shown at valueB1. Where photons of these different energy values can be differentiatedfrom each other, it is possible to identify one or both materials in thesame pixel or voxel image element of the obtained image. This same basicbehavior in response to radiation also allows some measure of capabilityto differentiate tissue types. By way of example, the graph of FIG. 8Bshows relative coefficients of attenuation for different bone densities.As FIG. 8B suggests, different linear absorption characteristics allowdifferentiation between various types of tissue, such as between bonetypes.

The use of multi-spectral or “color” x-ray imaging can have a number ofpotential benefits of value for dental, ENT, and head imaging. Theseinclude minimization of metal artifacts, separate reconstruction of softand hard tissue, more efficient segmentation algorithms for tooth andbone features, improved pathology detection for cancer and otherdisease, and detection of trace materials or contrast agents.

In addition to opportunities for improvement in the image processingchain, there are a number of differences in structure, operation,scanning sequence, dimensions, and supporting hardware that are neededto provide the advantages of photon counting in embodiments of thepresent invention. As one significant difference from conventionallarge-area image detection, the photon-counting architecture results inan image detector of reduced size, generally requiring a scanningsequence even where only a 2-D image is obtained. For volumetricimaging, such as in the sequence needed for CT or for cone-beam CT(CBCT) imaging, it may be necessary not only to scan within the sameplane, but to provide a 3-dimensional helical scan.

According to an embodiment of the present invention, a photon-countingsensor is provided as a retrofit to an existing panoramic imagingapparatus in order to provide additional capability, such as for CTimaging, for example. Retrofit can be performed by the operator asneeded by manually switching the detector type at the time of imageacquisition.

According to an embodiment of the present invention, the x-ray imagingsensor has an active area with a long dimension that exceeds its shortdimension by more than 1.5.

The photon-counting detector can be linear or rectangular, as well as ofsome other shape, such as an irregular shape, for example. Portions ofthe detector can be selectively enabled or disabled according to theimaging type that is needed. Thus, for example, an imaging sensor can berectangular in shape, but use only a line, slot, polygon, curved set ordesignated subset of pixels at a time for a particular type of sensing.Other shape variations are possible, with various portions of anirregularly shaped detector selectively enabled or disabled according tothe profile of the anatomy that is being imaged.

During exposure of the patient, the x-ray source can be pulsed orcontinuous and may change its exposure levels from one exposure to thenext. A helical or horizontal scan pattern can be used. Collimatormovement may be provided during the imaging sequence to direct radiationtoward the area of interest.

Setup, Alignment, and Positioning

As described with reference to FIGS. 1-3 , the patient or other subjectto be imaged is positioned between x-ray source 110 and x-ray detectorapparatus 120, as shown in more detail subsequently. As is familiar tothose skilled in the diagnostic imaging arts, a number of patientsupport devices, not specifically shown in FIG. 3 but described in moredetail subsequently, may also be provided for helping to stabilize andposition the head of the patient, including a chin supporting member,for example.

FIG. 9A is a schematic view of source-to-detector distances along anexposure path from x-ray source 110, at a position labeled O, that applyfor each type of imaging that can be performed by apparatus ofembodiments of the present invention. Three detector components withinrotary arm 140 are shown: a CT detector 122 at a distance Ob along theexposure path from x-ray source 110, a panoramic detector 124 at adistance Oc, and an optional cephalometric detector 126 at a distanceOd. Distances Ob, Oc, and Od can vary for each different type of imagingthat is performed, based on factors such as detector size, neededmagnification ratio, relative position of the subject, collimation, andother factors related to x-ray imaging. The relative position of asubject, shown as patient P, in the exposure path with respect to x-raysource 110 and to the various detectors 122, 124, and 126 is representedin dotted outline. The exposure path extends horizontally, in thex-direction as shown in FIG. 9 , along the rotary arm 140. Collimationat x-ray source 110 is used to substantially constrain exposureradiation to this linear path. An optional source translation apparatus112 can be provided for moving x-ray source 110 in the proper directionalong or orthogonal to the horizontal x-axis as shown. FIG. 9B shows anembodiment that is capable of moving the position of either source 110or detector 122 using a detector translation apparatus 113. Shownsubsequently are various arrangements of components that are used forpositioning the desired detector 122, 124, or 126 in place for each typeof imaging that is performed. As noted earlier, one or more of detectors122, 124, and 126 is a photon-counting detector according to anembodiment of the present invention.

Referring to FIGS. 10A-10D, there is shown an arrangement forpositioning, supporting, and moving the various CT, panoramic, andcephalometric detectors 122, 124, and 126 of FIG. 9 according to oneembodiment. FIG. 10A is a side view that shows a three-position detectorpositioning apparatus 130 with a movable platen 148 that is used tomount CT and panoramic detectors 122 and 124 adjacently, either back toback as shown in FIGS. 10A-10D, or side-by-side as shown in FIGS.11A-11D. In the context of the present disclosure, a platen isconsidered to be a single protruding support element that extends in adirection that is orthogonal to the length of rotary arm 140. Forreference, the relative position of rotary arm 140 is shown in dashedline form in FIGS. 10A and 11A. The platen itself could be in the formof a plate or other structure that provides one mounting surface or twomounting surfaces that are substantially in parallel. The platen ismovable as a single element to provide rotational or other curvilineartranslation of its corresponding detectors and could have variablethickness.

CT and panoramic detectors 122 and 124 mount back-to-back on a movableplaten 148 in the FIG. 10A embodiment. Movable platen 148, driven by adrive 132, rotates about a vertical rotation axis A1 to a suitableposition for each of the two or three imaging types. Axis A1 issubstantially orthogonal to the length of rotary arm 140, as shown inthe FIG. 10A embodiment. FIGS. 10B, 10C, and 10D are each top views,taken along rotation axis A1 to show detector positioning for each ofthree detector types. FIG. 10B shows a top view with movable platen 148of detector positioning apparatus 130 translated to a first position forCT imaging. In this configuration, CT detector 122 is properlypositioned on the direct path of, unobstructed with respect to, and inline with, x-ray source 110 at distance Ob. FIG. 10C shows a top viewwith movable platen 148 of detector positioning apparatus 130 rotated toa second position for panoramic imaging. In this next configuration,panoramic detector 124 is positioned at distance Oc along the exposurepath and is in the direct path of, unobstructed with respect to, and inline with x-ray source 110. FIG. 10D shows a top view with movableplaten 148 of detector positioning apparatus 130 moved to a thirdposition for cephalometric imaging, translated to displace detectors 122and 124 so that they are out of the exposure path between x-ray source110 and cephalometric detector 126. In this third position,cephalometric detector 126 is unobstructed with respect to, in thedirect path of, and in line with x-ray source 110. In the FIG. 10A-10Dembodiment, rotational translation of the movable platen between firstand second positions is with respect to a vertical axis or, moregenerally, to an axis that is orthogonal to the length of rotary arm140. Other sensor arrangements that allow rapid switching betweensensors include back-to-back configurations that rotate a shaft or otherdevice that runs between two back-to-back sensors.

In another embodiment, a radiographic imaging apparatus for obtaining animage based on received radiation (e.g., from a patient) can provide thecapability to obtain the image using two different imaging modalitiessuch as panoramic imaging and cephalometric imaging, for example of theslot-scan type, by sharing a single photon counting detector. Forexample, the single photon counting detector can be moved reciprocallybetween a first position to receive radiation generated by a panoramicimaging event and a second position to receive radiation generated by acephalometric imaging event. Such reciprocal movement can be implementedby an operator (e.g., physically moving the photon counting sensorbetween the first and second position), the operator using mechanicalapparatus or electro-mechanical apparatus, automatically (e.g., based onimaging modality selected) or the like. In one embodiment, differentportions of the single photon counting sensor (e.g, set of pixels) canbe used for different imaging modalities (e.g., slot having alength/width ratio>2 or a prescribed number, or approximately squarehaving a length/width ratio<1.4 or a prescribed number.

Referring to FIGS. 11A-11D, there is shown an alternate embodiment forpositioning, supporting, and moving the various CT, panoramic, andcephalometric detectors 122, 124, and 126. In this embodiment, detectors122 and 124 mount adjacently, such as side-by-side or top-to-bottom, onthe same side of movable platen 148. Movable platen 148 translatesdetector position relative to the plane of the platen, shown forreference as Q in FIG. 11A. FIG. 11A is a side view that shows athree-position detector positioning apparatus 134 having an x-ytranslation drive 136 for detector positioning. Detector positioningapparatus 134 provides a curvilinear translation path for the detectorsin a plane orthogonal to an axis A2 that is substantially parallel tothe length of rotary arm 140. FIGS. 11B, 11C and 11D are each top viewsshowing detector positioning for each of three detector types. FIG. 11Bshows a top view with movable platen 148 of detector positioningapparatus 134 translated to a first position for CT imaging. In thisconfiguration, CT detector 122 is properly positioned at distance Obalong the exposure path, unobstructed with respect to, and in the directpath of x-ray source 110. FIG. 11C shows a top view with movable platen148 of detector positioning apparatus 134 translated to a secondposition for panoramic imaging. In this configuration, panoramicdetector 124 is positioned at distance Oc along the exposure path,unobstructed with respect to, and in the direct path of x-ray source110. Distances Ob and Oc can be the same in this embodiment. FIG. 11Dshows a top view with movable platen 148 of detector positioningapparatus 134 moved to a third position for cephalometric imaging, withmovable platen 148 translated to remove detectors 122 and 124 out of thepath between x-ray source 110 and cephalometric detector 126 so thatcephalometric detector 126 is unobstructed with respect to, and in thedirect path of x-ray source 110. In the FIG. 11A-11D embodiment,curvilinear translation of the movable platen between first and secondpositions is in a plane that is orthogonal with respect to the length ofrotary arm 140. Curvilinear translation within the plane can be providedby a rotary actuator or by one or more linear actuators, for example.

Referring to FIGS. 12A-12D, there is shown another alternate embodimentfor positioning, supporting, and moving the various CT, panoramic, andcephalometric detectors 122, 124, and 126. Here, each of detectors 122and 124 are on separate movable platens 148. FIG. 12A is a side viewthat shows a two-position detector positioning apparatus 138 having anelevator apparatus 144 for detector positioning. Here, elevatorapparatus 144 is actuable to translate one or more of the detectors intoor out of the exposure path in a direction that is orthogonal to therotary arm. FIGS. 12B, 12C and 12D are each side views showing detectorpositioning for each of three detector types. FIG. 12B shows a side viewwith detector positioning apparatus 138 supporting detectors in a firstposition for CT imaging. Here, CT detector 122 is properly positionedunobstructed with respect to, and in the direct path of x-ray source 110along the exposure path at distance Ob. FIG. 12C shows a side view withelevator assembly 144 of detector positioning apparatus 138 actuated tolift CT detector 122 out of the exposure radiation path to allowpanoramic imaging. Here, panoramic detector 124 is positioned along theexposure path at distance Oc, unobstructed with respect to, and in thedirect path of x-ray source 110. FIG. 12D shows a side view withelevator assembly 144 of detector positioning apparatus 138 actuated tolift panoramic detector 124 up and out of the exposure radiation path toallow cephalometric imaging. Both detectors 122 and 124 are translatedby elevator 144, out of the path of exposure radiation between x-raysource 110 and cephalometric detector 126, so that cephalometricdetector 126 is unobstructed with respect to, and in the direct path ofx-ray source 110 on the exposure path at distance Od.

Embodiments of the present invention also use the photon-countingdetector in partial CT scanning mode. Unlike full CT scanning, which canexpose the patient to considerable amounts of radiation for obtainingvolume image content, partial CT scanning uses a cone beam of reducedsize and scans over a small range of angles to provide the volume data.Relative to the imaging apparatus used, partial CT imaging restricts thegenerated radiation from portions of the anatomy that are outside of theimage area. In this way, partial CT is a more localized imaging mode,well suited to imaging a single tooth or group of adjacent teeth orother adjacent structures. Referring to FIG. 12E, there is shown part ofa scan pattern for partial CT scanning, with a source 162 directing anarrowed cone beam of radiation to a photon-counting detector 164through a portion of an arch 160.

Rotation for partial CT imaging is typically over a small range ofangles, such as that shown as angle a in FIG. 12E. Angular rotation ofas little as 5 degrees can be used for partial CT image acquisition.Other modifications to standard CT imaging include the use of cone beamradiation, typically modulated by providing the beam through a slit ornarrowed collimator path, for example, instead of conventional fan beamradiation. Radiation can be continuously provided throughout the scan orpulsed, sensed at discrete angular intervals. The images acquired atdifferent angles during the partial CT imaging sequence generally coveronly a small area with this imaging modality. Rotation about a centralaxis, as shown in FIG. 12E, simplifies image processing andreconstruction.

According to an embodiment of the present invention, a slit or othermechanism is used to narrow the cone beam dimensions appropriately forpartial CT scanning. The length of the slit corresponds to the lengthdimension of the corresponding detector 164; the overall shape of thedirected beam of radiation is designed to be compatible with the aspectratio of the photon-counting detector. Synchronous movement of detector164 and source 162 thus allow volume image data to be acquired over areduced range of angles using this imaging mode.

For partial CT imaging, a series of x-ray projection images is produced.Images that have been generated using the conical projection beam arestored and processed to obtain volume image data. Projection processingand methods used for 3D volume image reconstruction are well known tothose skilled in the optical imaging arts.

Each of the embodiments shown in FIGS. 10A-12D allows a measure ofautomation for setting up the proper detector in each position and fordetermining when the detector is suitably positioned so that imaging canproceed. For example, operator commands entered at an operator console(not shown) can be used to set up a second imaging type after a firstimage is obtained. Optionally, operator controls on rotary arm 140 canallow the imaging configuration to be shifted from one imaging type toanother. Manual positioning may also be used, or some combination ofmanual and automated actuation for achieving each configuration.Different types of imaging sensors can be raised from or lowered intothe path of incident radiation for various imaging types, for example. Amovable carriage or other device can be used to switch the appropriatedetectors into place as needed and to move unwanted detectors out of thepath of radiation. When moved out of the imaging path, the imagingsensors may be shielded from exposure to radiation or to dirt.

According to an embodiment of the present invention, the same sensor isused for multiple types of imaging. Sensor response is treateddifferently for each imaging type. This can include windowing and otherfeatures that control how the exposure data is acquired for each imagingtype. That is, the ranges of pixels that are read for each imaging typecan be different.

Collimation can be used to control scanning of the exposure onto thedetector. A slit, for example, can be formed and used for directing theradiation only along the slit, as the slit is scanned with respect tothe sensor surface. Still other types of windowing may be used,including those that use the collimator or digital windowing algorithmsthat select the size of the area of the imaging sensor from which datais to be acquired. Image enlargement ratios can be adjusted according tothe type of image data that is being obtained.

Offset detector geometry can be advantageous in some applications likeCT imaging. The rectangular or roughly square shaped sensor is orientedin such a manner that the longitudinal axis extending from the generatorto the sensor and passing through the rotation axis is perpendicular tothe active surface of the sensor, the center of the sensor being offsettransversely relative to the projection of the axis onto the activesurface of the sensor. With photon-counting sensors, for example, asensor offset or eccentricity can help to provide additional depthinformation for the scanned subject. For example, thanks to such anoffset arrangement, on moving the ensemble source and photon countingsensor around the object in a full turn, a greater lateral extend of theobject is captured. According to an embodiment of the present invention,the central axis of the radiation beam does not intersect the rotationaxis.

Still other types of adjustment that are possible according toembodiments of the present invention include adjustment of a collimator104, positioned in front of X-ray source 110 as shown in FIG. 9 . TheField of View (FOV) of the imaging apparatus can be adjusted by changingthe relative positions of the x-ray source and sensor as well as bymaking appropriate changes to tube voltage and current, as well as bycollimator 104 adjustment.

The size of the FOV can be switched in a number of ways. According toone embodiment of the present invention, an operator interface on acontrol monitor allows selection of an FOV size. This selection thenchanges appropriate X-ray conditions (such as tube voltage and tubecurrent, for example) according to the specified FOV size. A motorcoupled with the collimator can also be adjusted to change the amount oflight that is obtained by the imaging sensor or camera. As the FOV sizeis reduced with the same X-ray conditions, the light amount output tothe imaging sensor decreases; proper collimator adjustment can help toremedy this problem.

As one example, the FOV can be changed between imaging of the whole jawand imaging of a tooth ridge and nearby bones of one tooth. Here, theFOV size of X-rays is initially set for the whole jaw, a tomographicimage is acquired of the whole jaw. Then, following adjustment of theFOV, a tomogram of one tooth is obtained and displayed. In this manner,when the whole jaw and one tooth are sequentially imaged, the resolutiondoes not deteriorate, so that an image usable for assessment anddiagnosis can be obtained. According to an alternate embodiment of thepresent invention, the FOV is adjusted automatically following operatorselection of an imaging mode.

Rotation Axis Positioning Relative to the Patient and Imaging Apparatus

A number of different apparatus and methods can be used for positioningthe patient relative to the axis of rotation of the x-ray apparatus. Achair or other device can be used to support the patient in position, sothat the rotation axis can be adjusted accordingly. Various types of earrods, head straps and supports, chin rests, bit blocks, and otherdevices can be used to position the head of the patient and to restrictpatient movement. The various devices that are used can be employed inany of a number of combinations for constraining head movement andproviding a reference for axis positioning.

According to an embodiment of the present invention, one or more lightbeams are used to indicate positions of the patient anatomy. The lightbeams can be sensed by one or more electronic sensors. According to analternate embodiment of the present invention, light beams forhorizontal and vertical alignment are directed toward the face and headof the patient and used as reference marks to guide head positioning. Inthe schematic view of FIG. 9 , a light source 114 is used to providethis function, providing registration marks that enable the patient tobe appropriately positioned for the type of imaging that is beingperformed. This can include, for example, reference positioning of theFrankfort plane, or other anatomical guideline. The use of guide beamsin this way can provide information needed for automatic centering oradjustment of the rotation axis relative to the subject patient. Forexample, a vertical beam can correspond to a center (e.g., prescribedposition, arc or boundary) of a region selected for imaging. Asupporting display can be used in conjunction with an axis positioningsystem, enabling the practitioner to verify that the rotation axis issuitable for the anatomy that is being imaged.

The depth of the region around a selected scan curve that is ineffectively sharp focus is known as the “focal layer thickness” or“focal trough.” For dental imaging, where the panoramic view can be aperspective view of the dental arch with detail in the vertical andcircumferential directions, or a portion thereof (e.g., as imaged fromthe inside or the outside), a view, but considerable focal trough depthin the radial direction, perpendicular to the plane of the image, isfrequently desirable. Data from object features outside the effectivefocal trough are “smeared” out over different image pixels to such anextent that they contribute little to the final image. The wider thestrip of sensors that is used to sum the image points, the more rapidlythe data will cease to be related and become smeared as the distancefrom the selected image point at the center of the focal troughincreases, and thus the smaller is the depth of focus of the finalimage.

Detection of the focal trough for panoramic imaging using thephoton-counting detector can be performed in any of a number of ways.According to embodiments of the present invention, measurements of thedental arches are obtained to determine an axis for revolution of theX-ray source and sensor. Alternately, light beams are used to providethe best estimate for this axis, based on an estimate of the focaltrough. The axis of rotation can also be adjusted during a panoramicimaging sequence, so that the movement of the axis of rotationcorrelates with the horseshoe-shaped pattern of the focal trough. In theexample schematic of FIG. 9 , for example, source translation apparatus112 is actuated during the scan, moving the x-ray source 110 duringrotation of rotary arm 140, effectively changing the axis of rotation ina continuous or discrete fashion as images are acquired. Defaultmodeling of focal trough position and dimensions can alternately beused.

Focus depth for a region of interest can be calculated in a number ofways. According to an embodiment of the present invention, focus depthwhich is different in a region of interest can be distinguished from afocus depth corresponding to a predetermined panoramic image, whereinthe focus depth for the region of interest is determined automatically,wherein the focus depth for the region of interest is determined by theprocessing device automatically by: calculating multiple layers using alaminographic reconstruction of the plural layers; calculating a measureof sharpness for each of the multiple calculated layers; and based onthe calculated sharpness measure, choosing the sharpest layer from amongmultiple calculated layers for the region of interest to provide for anddisplay a corrected layer for the region of interest defined for thepanoramic image as a predetermined anatomical region defined by apredefined geometric path of the single source and detector, patienttype, and a predetermined speed profile.

Focus depth for panoramic imaging and other imaging types can also bevaried in a number of ways when using a photon-counting detector.According to an embodiment of the present invention, a user-identifiedregion of interest (ROI) along a particular focal trough is defined. Avariable focal pattern is used in the imaging scan, in order to obtainimage data at different focal regions, indicative of tissue depth. Forimaging a particular ROI, the corresponding image data can be obtainedfrom the varied focal pattern used for acquiring the image content. Thefocus depth for an ROI for a panoramic or volume image can beautomatically determined and can be different from the focus depth usedfor other images in the set of images obtained from the same patientduring the same exam.

The schematic diagram of FIG. 13 shows an imaging apparatus 200 forradiographic imaging, such as panoramic imaging, in which a successionof two or more 2-D images is obtained and images of adjacent content arearranged to form a larger image, or for 3-D imaging, such as tomography,computed tomography volume imaging, or cone beam computed tomography(CBCT) imaging in dental, ENT, and related head imaging applications. Arotatable mount 230 is provided on a column 218, preferably adjustablein height to suit the size of patient 12. Mount 230 maintains x-raysource 110 and a radiation sensor 121 on opposite sides of the head ofpatient 12 and, optionally, rotates to orbit source 110 and sensor 121in a scan pattern about the head. Mount 230 rotates about an axis A2that corresponds to a central portion of the patient's head, so that itsattached components orbit about the head. Sensor 121, a photon-countingsensor according to an embodiment of the present invention, is coupledto mount 230, opposite x-ray source 110 that emits a radiation patternsuitable for 2-D imaging, for tomosynthesis imaging, or for CT or CBCTvolume imaging. Multiple sensors 121 can alternately be provided, asdescribed previously, with the suitable sensor switched into place foreach particular type of imaging. An optional head support 236, such as achin rest or bite element, provides stabilization of the patient's headduring image acquisition.

A computer 106 has an operator interface 204 and a display 108 foraccepting operator commands and for display of volume images obtained byimaging apparatus 200. Computer 106 is in signal communication withsensor 121 for obtaining image data and provides signals for control ofsource 110 and, optionally, for control of a rotational actuator 212 formount 230 components.

One or more height sensors 234 is also sensed by computer 106 in orderto obtain an initial height setting and to track relative verticaldisplacement of the sensor 121 relative to the patient's head during thehelical scan. Computer 106 is also in signal communication with a memory232 for storing image data. An optional alignment apparatus 240 isprovided to assist in proper alignment of the patient's head for theimaging process. Alignment apparatus 240 includes a laser that providesone or more line references for head positioning according to anembodiment of the present invention. The alignment provided can behorizontal, vertical, or may specify position and angle, for example.Lines can display on the head or body of the patient. Alternately, lightfrom the laser or other light source can be directed toward aphotosensor element. In alternate embodiments, alignment apparatus 240includes a visible light beam or other marker, or a mechanical or otherpositioning apparatus. Imaging apparatus 200 may also have thecapability for panoramic or cephalometric imaging using x-ray source 110and sensor 121 or other imaging sensor.

There can be a number of variable scan patterns according to the type ofimaging that is required. Tomosynthesis, for example, typically uses ascan that revolves over an arc of less than 180 degrees about thepatient. CBCT scanning may require a helical scan pattern with one ormore revolutions about the patient's head. An optional adjustmentmechanism 242 is provided for adjusting the source-to-image (SID)distance between the x-ray source 110 and sensor 121 to suit the scanpattern for different patients or different types of imaging.

An alternate scan pattern for extra-oral imaging shifts the position ofthe axis of rotation during the scan. This pattern, changing the focaltrough progressively during the scan by adjusting the axis of rotation,allows improved imaging of different portions of the dental arch.According to an embodiment of the present invention, the location of theaxis is changed with respect to two dimensions, that is, with respect toboth x- and -y axes in the x-y plane that is normal to the axis ofrotation. Changes in position can be along a line in the x-y plane, oralong a curvilinear path, for example.

Multiple images can be obtained from a single exposure, using varioustechniques, with and without the thresholding capabilities of thephoton-counting imaging detector.

One drawback of typical photon-counting image detectors is theirrelatively small size. Unlike a conventional digital radiography imagingpanel that has an array with hundreds of elements in the height andwidth directions, the photon-counting sensor is typically of smallersize, with a width that may be fewer than 100 pixels in dimension. Thisproblem can be addressed by tiling, an approach in which multiple imagedetectors are combined to cover a larger detection area. The use ofpolycrystalline materials, as opposed to conventional monocrystallinedetector materials as noted earlier, can also help to provide largerdetectors.

Another solution for the size constraints of photon-counting imagedetectors adapts their scanning sequence to effectively increase thefield of view. In practice, this size limitation requires a differentscanning sequence from that used for conventional CBCT imaging. Ahelical scan can be used to acquire the needed image data for volumeimaging. In operation, mount 230 rotates about the head of patient 12multiple times, thereby scanning sensor 121 about patient 12 in ahelical orbit, as is shown in FIG. 14 . In FIG. 14 , an adjacent imagingposition is shown in dotted outline, with the angular distanceexaggerated for clarity. According to an embodiment of the presentinvention, the vertical height h change of the helix during revolutionof the source and detector, which can also be expressed in terms of thehelical pitch angle P1, and angular change θ between successive imageacquisitions, is adjustable.

The helical scan needed for CBCT imaging using a photon-counting sensor121 can be provided following either of a number of scanning apparatusmodels. FIGS. 15A and 15B show a first approach to this problem, inwhich mount 230 that contains sensor 121 and source 110 is itselfcoupled to a movable travel arm 228 that is vertically translated duringthe scan, displaced by an actuator 212 during rotation of mount 230.This translation changes the relative vertical position of the imagingsensor and the radiation source to the patient's head during the helicalscan.

FIGS. 16A and 16B show a second approach to this problem, in which mount230 itself has the same height, while source 110 and sensor 121 arevertically translated during the helical scan, thereby changing therelative vertical position of the imaging sensor and the radiationsource to the patient's head during the helical scan. FIGS. 17A and 17Bshow a third approach to this problem, in which mount 230 itself has thesame height, while a vertically adjustable platform 238 is used as anactuator to provide relative movement between the head of the patientand source 110 and sensor 121 for changing the relative verticalposition of the imaging sensor and the radiation source to the patient'shead during the helical scan.

As shown in FIGS. 15A-17B, one or more actuators 124 within mount 130,or other height adjustment devices provide this vertical translationfunction as source 110 and sensor 121 revolve about the patient's head.Computer 106 coordinates and tracks the vertical and rotational orangular movement and corresponding actuators needed for helicalscanning. Sensor 134 provides feedback information on height with theFIG. 15A/B, FIG. 16A/B and FIG. 17A/B scan configurations.

Scanning can advance the position of the imaging sensor by less than apixel between consecutive image frames.

The logic flow diagram of FIG. 18 shows an operational sequence for CBCTscanning of the head using a photon-counting detector according to anembodiment of the present invention, for the imaging apparatus shown inFIGS. 14, 15A, 15B, 16A, 16B, 17A, and 17B. In an accept instructionstep S210, the imaging apparatus accepts operator instructions relatedto the type of image to be obtained, which may include information onthe types of tissue that are of particular interest. In a thresholdsetup step S220 an appropriate set of threshold values and otheroperational parameters is loaded to circuitry of sensor 121. An operatorsetup step S230 allows the operator to adjust mount 230 components tosuit the height of the patient and size of the patient's head. Thisregisters an initial height setting that provides information forsubsequent helical scan execution. The operator can also use headsupport 236 and alignment apparatus 240 to adjust patient position. Aninstruction entry step S240 accepts the operator instruction to beginthe scan sequence and to execute a scan and acquisition step S250.During step S250, multiple 2-D images are obtained at successiverotational and height positions for acquiring the CBCT scan data. Animage generation step S260 then forms the 3-D volume image from theobtained 2-D images, using an image reconstruction algorithm, such asone of the filtered back-projection routines well known in the volumeimaging arts. The resulting image is then displayed for viewing by thepractitioner and the image data is stored in memory 232 (FIG. 13 ) orother memory circuitry that is accessible to computer 106.

According to an embodiment of the present invention, the tissue type ofinterest dictates the set of operational parameters that are mostsuitable for imaging a particular patient. By way of example, and not byway of limitation, Table 1 lists a set of parameters that are loadedwhen the operator elects to generate an image for tissue type A. Table 2lists alternate example parameters for generating an image for tissuetype B. As described earlier with respect to FIG. 14 , the pitch of thehelical scan pattern can be specified in terms of vertical translationor helical pitch angle P1. The helical pitch angle P1 can be varied from0 degrees (that is, a slope of 0) to 40 degrees or more and is based onthe relative size of the sensor 121 and the amount of overlap neededbetween successive images.

It can be appreciated that some modification of procedures listed anddescribed with reference to FIG. 14 are similarly used for other typesof imaging using imaging apparatus 100, with appropriate changes for thescan pattern and number of images obtained. For panoramic ortomosynthesis imaging, for example, a full scan is not needed. Only apartial scan is needed, with the scan orbit defined within a singleplane, rather than helical as described for CBCT scanning.

TABLE 1 Operational Parameters for Tissue Type A Parameter SettingRadiation energy level 30 kVp Threshold values to sensor +1.23 V +1.41 VImage acquisition interval every 0.8 degrees Vertical translationbetween images 0.1 mm

TABLE 2 Operational Parameters for Tissue Type B Parameter SettingRadiation energy level 40 kVp Threshold values to sensor +1.02 V +1.34 VImage acquisition interval every 0.9 degrees Vertical translationbetween images 0.12 mm

As noted earlier, different types of materials have different photonenergy “signatures”, enabling the volume scan to detect two or moredifferent materials in the imaged subject. This feature enables the sameimaging apparatus to be employed for obtaining different informationusing the same scanning pattern. According to an embodiment of thepresent invention, different sets of threshold settings are provided,depending on the nature of the volume image that is desired. One set ofthreshold settings, for example, is optimized for obtaining informationon teeth, while another set of threshold settings works best for imaginggum and underlying support structures. Still another set of thresholdsettings provides the best conditions for imaging of the throat, ear, ornasal passages, with corresponding elevation adjustments. As describedwith reference to FIG. 18 , an appropriate set of threshold values isselected and loaded to the image acquisition circuitry of the imagingsensor according to the type of imaging that is to be performed and tothe type of tissue that is of particular interest for a patient.

Embodiments of the present invention have been described for imagingvarious regions of the head and upper body of a patient using anextra-oral detector. The apparatus of the present invention can be used,for example, to obtain a full-mouth series (FMS) in dental practice. Itshould be noted that sensor 121 (FIGS. 13, 14 ) can combinephoto-counting circuitry with other, conventional imaging components,such as with indirect detection or integrating imaging componentsdescribed earlier with reference to FIGS. 4A-4D. Multiple sensors 121can be coupled together to increase the area over which an image isobtained for each x-ray exposure. The photon-counting sensor 121 can beused to support different imaging modes, including CT or CBCT,panoramic, or cephalometeric imaging. CT and CBCT imaging modes obtain avolume image from multiple 2-dimensional (2-D) images. Panoramic andcephalometeric imaging are 2-dimensional imaging modes that may requirescanning of sensor 121 in one or two directions within the same imagingplane during imaging in order to cover the full imaging area.

With the necessary adaptations to hardware and to the scanning patternsthat are used, embodiments of imaging apparatus 200 (FIG. 13 ) arecapable of a number of types of imaging, including 2-D imaging andpanoramic imaging, tomosynthesis imaging, and volume imaging usingcomputed tomography (CT) or cone-beam computed tomography (CBCT).

Tomosynthesis is an imaging mode that takes advantage of the capabilityof systems such as imaging apparatus 200 to localize focus over somefractional portion of an arc and to process the resulting image data inorder to provide an image that provides some amount of depth informationfrom a series of individual 2-D images obtained at different anglesalong the arc. Tomosynthesis thus provides a type of volume image withlimited depth information, formed from a sequence of two-dimensional(2-D) images. Basic principles for dental tomosynthesis are known in thedental imaging arts and are described, for example, in U.S. Patent No.5,677,940.

One drawback of tomosynthesis techniques using photon-counting detectorsrelates to the discrepancy that can occur between the focus layer andthe actual region of interest, such as the patient's teeth.Discrepancies can occur even when the focal layer (e.g., spline, locusof the rotation axis) is predefined for a given region along the dentalarch or other structure. However, this disadvantage can be addressed orremedied by permitting the choice of a different, iteratively selected,or best focus layer that is different from the preset layer (e.g., forone or more portions of the preset layer) and by adapting the positionof this best focus layer relative to the shape of the patient's dentalarch. In processing, a shift of pixels within each image is performed,the amplitude of the shift chosen so that the position of the anatomicalstructure of interest is located, after shifting, at the same positionon each image. After a pixel-to-pixel adding process of the plurality ofacquired images, a final image is obtained in which the anatomicalstructure of interest is located in the focus layer and other structuresare blurred (resulting in horizontal stripes, for example). By repeatingthe process with other shift amplitudes values, a plurality of focuslayers can be obtained and the best one can be chosen for a region ofinterest. Among advantages of this technique are image quality, which isonly slightly dependent upon the positioning of the patient.

It should be noted that extra-oral embodiments of the present inventioncan also provide an analog count, rather than using a digital counterarrangement. The accumulated analog charge, incremented once for eachphoton that is received, can be distinguished from conventional types ofintegrated radiation detection that provide a digital value according tothe relative brightness of each pixel in the scintillator.

The scan curve can be altered by the user either interactively, such asduring scanning and image acquisition, or following a particular scansequence (e.g., at the image acquisition apparatus or by a viewer), suchas with a panoramic image, for example.

Patient Positioning and Stabilization

Patient support apparatus help to provide fixed reference positioning ofthe patient relative to the imaging system for obtaining a volume imagewhen using a photon-counting sensor. The schematic views of FIGS. 19,20A and 20B show features of a head support apparatus 370 at a restposition according to an embodiment of the present invention. Headsupport apparatus 370 mounts on a support structure 310 and has a mainbody, a base, and an optional chin rest 302. Temporal holding members304 and 306 are positioned against the patient's head, each coupled to atransport apparatus, 372 and 374 respectively. Each holding member 304and 306 has a respective reference position, shown as R1 and R2,respectively. Components of transport apparatus 372 and 374 include oneor more shafts or other elements that extend outward, one from each sideof a main body 312. Ear rods 340 and 350 are provided for seating withinouter portions of the patients' ear cavity. In the rest position shownin FIG. 19 , the distance between both ear rods 340 and 450 at the restposition is significantly smaller than the size of a patient's head. Arotatable gantry 360, also mounted on support structure 310, holds x-raysource 110 and sensor 121 for detecting x-rays and forming an image.Supporting structure 310 is representative in FIG. 19 and can take anynumber of forms for a standing or seated patient. Not shown are otherconventional support components of the x-ray imaging apparatus, such asrotation mechanism for gantry 360 rotation, for example. A mouthpiece orbite structure could also be provided to help in further stabilizing thepatient's head or in adjusting the head angle. As described in moredetail subsequently, the position of axis of rotation A1 could vary,depending on whether reference position R1 or reference position R2 isfixed.

An extension locking mechanism 380 constrains movement of either oftemporal holding members 304 and 306 at a time. As shown in FIG. 20A,when holding member 306 is moved outward from main body 312, holdingmember 304 is locked in its reference position R1. Similarly, as shownin FIG. 20B, when holding member 304 is moved outward from main body312, holding member 306 is locked in its reference position R2. In thisway, only one of holding members 304 or 306 can be moved outward at onetime. At any one time, either holding member 304 is at referenceposition R1 or holding member 306 is at reference position R2. Thisallows alternative references for supporting patient head H in positionrelative to an axis of rotation A1 or A2 for gantry rotation of thex-ray source and sensor during imaging.

In one embodiment of the present invention, the axis of rotation can beset to either of two positions, shown as A1 and A2 in FIGS. 20A and 20B,allowing repositioning of the imaging gantry for imaging structures ofthe left or right ear when rotating along different axes, for example.Adjustment to position for axis A1 or A2 can be a mechanical adjustmentmade by an operator or may be automatically performed by control logicthat operates the volume imaging apparatus. An optional sensor 334, asshown in FIG. 20A, enables control logic for the volume imagingapparatus to determine whether or not holding member 304 or 306 has beenadjusted, so that proper axis selection can be enabled. Holding members304 and 306 support corresponding ear rods 340 and 350 respectively. Thevertical position of the chin support relative to the main body isadjustable and relative to the ears of the patient. When properlypositioned using head support apparatus 370, the patient can be held onthree non planar points: at the chin and at two ears, so that thepatient's head is stabilized and does not move during imaging. Ears ofthe patient are in a well-defined position relative to the gantry andits related imaging components. It should be noted that the patient'shead position can be different based on which holding member 304 or 306is adjusted.

As another alternative for patient support, a mask can be used, as shownin perspective views of FIGS. 21 and 22 . Mask 386, as shown in FIG. 21, has handles 352 that allow the patient to grasp mask 386 for supportand a window 378 for improved patient visibility. FIG. 22 showscomponents for supporting the head of the patient in position on mask386. A chin rest 382 and bite support 384 are provided. Window 378allows the patient to breathe more comfortably and allows bettervisibility for the patient and practitioner during setup. Mask 386 canbe fully or partially transparent for even better visibility. Certainexemplary embodiments of patient supports can further providepositioning guidelines or marks (e.g., horizontal, vertical, alignment,spatial relationships) to assist an operator. An adjustable headrest 390is also provided.

Control Functions

Various control functions are provided that allow setting of parametersand, where available, selecting one of multiple imaging types. Thus, forexample, an operator can select image scanning and acquisition for CT,tomographic, 2-D, or panoramic imaging mode. In an alternate embodiment,the operator can specify an appropriate axis of rotation. Theseselections can be made on interface 204 (FIG. 13 ) or on some othercontrol console, for example. Execution of operator instructions thencauses the imaging apparatus to configure its arrangement of sensors fordifferent imaging modes as well as to configure other aspects of systemoperation, such as collimator settings, voltage threshold levels, focusdistance, and other parameters.

Image Processing and Reconstruction

For depth imaging when obtaining CT, panoramic, or other images, a setof images with multiple 2-D images is first obtained and stored. Imageswithin the obtained set of images differ from each other according tothe angle of imaging relative to the patient. In addition, images withinthe set acquired for a patient can also differ in radiation energylevels used, threshold energy levels detected, collimator positions andsettings, region of the detector over which data is obtained, focusdistance, and other characteristics.

A number of options are available for reconstruction of a volume imagefrom individually obtained 2-D images when using a photon-counting imagedata sensor. Conventional methods for volume image reconstructioninclude the Feldkamp-Davis-Kreiss (FDK) method, an analyticalreconstruction method that uses filtered back-projection to construct a3-D image from individual 2-D slices. An alternate method, particularlyuseful where a limited number of 2-D image projections are available, isAlgebraic Reconstruction Technique (ART) that iteratively solves asystem of linear equations whose unknowns are corresponding image datavalues.

The focal trough of a panoramic image can be defined by parameters suchas the trajectory of the axis of rotation during the scan. Then, apanoramic image can be reconstructed along a predetermined layer. Usingthe shift and add technique, a plurality of panoramic images can bereconstructed, on the basis of the same set of scan data (e.g., using aplurality of values of the shift). A panoramic image can also containparts that are constructed using various shift and add values, thenhaving various focal troughs. Such a construction can provide a goodsharpness of the whole image or portions thereof, independent of theshape of the dental arch.

According to an embodiment of the present invention, reconstruction ofthe image data at different focal troughs can be performed using thedata from a single scan. To obtain the image data during the scan, thefocal trough can be continuously changed during the scan by adjustingthe axis of rotation. Alternately, the focal trough can be switchedduring the scan in order to obtain image data from different portions ofthe dental arch. Switching of the focal trough can be performed, forexample, by changing the position of the axis of rotation as well as bychanging the effective focal position of the imaging apparatus.

Embodiments of the present invention provide a real time storage andretrieval of the collected and irradiated frames with a short time, suchas with a delay of not more than one second. The reconstructionalgorithm can reconstruct a panoramic layer or other depth image contentfrom the stored and retrieved irradiated frames and display a panoramiclayer or other feature in no later than 10 seconds from the end of anexposure series.

A shift-and-add algorithm, known in the art, can be used for imagereconstruction. For this type of processing, the position of the currentframe is recorded as a coordinate in the final image. This coordinate isthen used to calculate an amount of shift required in a shift-and-addalgorithm for reconstructing the final image. Sub-pixel shifting isaccomplished by adding the pixels in a frame to two locations in thefinal image multiplied by suitable weighting factors. If the targetposition is x (non-integer or integer) and the position is increasing ina positive direction, then the pixel value is added to positionsfloor(x) and ceil(x) where floor(x) refers to the largest integersmaller than x and ceil(x) refers to the smallest integer larger than x.The respective weighting coefficients w_(left) and w_(right) arex-floor(x) and ceil(x)-x. The weighting factors w_(left) and w_(right)can be global to a single frame or can vary from pixel to pixel tocompensate for any time delays between individual pixels.

This is mathematically equivalent to interpolating the frames and finalimage linearly in the horizontal direction, shifting the frame pixels inthe horizontal direction by an integer amount and then down-sampling theframes and the final image to the original size. The sub-pixel shiftingcan also be implemented using any other interpolation method, forexample with bi-linear, bi-cubic or spline interpolation.

An algorithm is provided which auto-focuses a panoramic layer andautomatically calculates the layer-of-best-focus for dental panoramicimaging. The algorithm uses multiple image frames to compose a panoramicimage of a layer of the object under observation, the image having afocus depth which is different in at least some part of the panoramicimage from the focus depth corresponding to a predetermined panoramicimage. In a first step, frame data is used to reset the change invelocity Δv of movement in the image plane compared to the change inoriginal velocity Δv_(orig). In a second step, the user specifies aregion of interest. In a third step, the region of interest isreconstructed at the original speed V_(orig) plus the change invelocity, Δv. In a fourth step, the sharpness measure S_((n)) (sharpnessmeasure S, which can either be a measure of contrast, roughness or someother measure of the image sharpness) and sharpness difference. ΔS iscalculated as being equal to S_((n)) minus S_((n−1))SM(V_(orig)+Δv). Ina fifth step, if ΔS is less than a particular limit, then the region ofinterest is displayed; otherwise, calculation uses a different stepdelta velocity Δv and returns to the third step and continues. Thealgorithm can be applied globally to the whole final image or locally toa given region-of-interest. Therefore the dentist is able to observe aninitial panoramic image and then select a region (portion) of the imagewhere blurring may be evident, in which case, the algorithm maximizessharpness S of the selected part of the image. The result is a completeimage with all parts well in focus.

Methods such as spine compensation can be used for providing images ofimproved quality for assessment of dental structures. Subtraction ofblurred image content is used to provide this compensation, according toan embodiment of the present invention.

For the various embodiments shown, signal communication betweencomponents can use wired or wireless signal channels. The use ofwireless communication can be advantaged, for example, for receivingsignals from sensor devices that are difficult to maintain connectionto. Wireless communication can also be used between sensor 121 and theimage processor that obtains image data. Referring to FIGS. 15A and 15B,for example, sensor 121 communicates the acquired image data to computer106 using wireless transmission according to an embodiment of thepresent invention. It should be noted that high transmission speeds arerequired for wireless transmission of image data, particularly forvolume imaging applications that require that multiple 2-D images beobtained in order to reconstruct the 3-D image.

Consistent with an embodiment of the present invention, a computerexecutes a program with stored instructions that perform on image dataaccessed from an electronic memory. As can be appreciated by thoseskilled in the image processing arts, a computer program of anembodiment of the present invention can be utilized by a suitable,general-purpose computer system, such as a personal computer orworkstation, as well as by a microprocessor or other dedicated processoror programmable logic device. However, many other types of computersystems can be used to execute the computer program of the presentinvention, including networked processors. The computer program forperforming the method of the present invention may be stored in acomputer readable storage medium. This medium may comprise, for example;magnetic storage media such as a magnetic disk (such as a hard drive) ormagnetic tape or other portable type of magnetic disk; optical storagemedia such as an optical disc, optical tape, or machine readable barcode; solid state electronic storage devices such as random accessmemory (RAM), or read only memory (ROM); or any other physical device ormedium employed to store a computer program. The computer program forperforming the method of the present invention may also be stored oncomputer readable storage medium that is connected to the imageprocessor by way of the internet or other communication medium. Thoseskilled in the art will readily recognize that the equivalent of such acomputer program product may also be constructed in hardware.

It will be understood that the computer program product of the presentinvention may make use of various image manipulation algorithms andprocesses that are well known. It will be further understood that thecomputer program product embodiment of the present invention may embodyalgorithms and processes not specifically shown or described herein thatare useful for implementation. Such algorithms and processes may includeconventional utilities that are within the ordinary skill of the imageprocessing arts. Additional aspects of such algorithms and systems, andhardware and/or software for producing and otherwise processing theimages or co-operating with the computer program product of the presentinvention, are not specifically shown or described herein and may beselected from such algorithms, systems, hardware, components andelements known in the art.

It should be noted that the term “memory”, equivalent to“computer-accessible memory” in the context of the present disclosure,can refer to any type of temporary or more enduring data storageworkspace used for storing and operating upon image data and accessibleto a computer system. The memory could be non-volatile, using, forexample, a long-term storage medium such as magnetic or optical storage.Alternately, the memory could be of a more volatile nature, using anelectronic circuit, such as random-access memory (RAM) that is used as atemporary buffer or workspace by a microprocessor or other control logicprocessor device. Display data, for example, is typically stored in atemporary storage buffer that is directly associated with a displaydevice and is periodically refreshed as needed in order to providedisplayed data. This temporary storage buffer can also be considered tobe a memory, as the term is used in the present disclosure. Memory isalso used as the data workspace for executing processes and forrecording entered values, such as seed points, or storing intermediateand final results of calculations and other processing.

Computer-accessible memory can be volatile, non-volatile, or a hybridcombination of volatile and non-volatile types. Computer-accessiblememory of various types is provided on different components throughoutthe system for storing or recording, processing, transferring, anddisplaying data, and for other functions.

The disclosed exemplary embodiments are considered in all respects to beillustrative and not restrictive. In addition, while a feature(s) of theinvention can have been disclosed with respect to at least one ofseveral implementations/embodiments, such feature can be combined withone or more other features of other implementations/embodiments as canbe desired and/or advantageous for any given or identifiable function.

1. A dental imaging apparatus for obtaining an image from a patient, the apparatus comprising: a radiation source; a first digital imaging sensor that provides, for each of a plurality of image pixels, at least a first digital value according to a count of received photons that exceed at least a first energy threshold; a mount that supports the radiation source and the first digital imaging sensor on opposite sides of the patient's head; a computer in signal communication with the first digital imaging sensor for acquiring a first two-dimensional image from the first digital imaging sensor; and a controller electrically coupled to the radiation source, the first digital imaging sensor, the mount and the computer, the controller configured to operate the dental imaging apparatus in at least one imaging mode.
 2. The apparatus of claim 1, where the dental imaging apparatus is configured to operate in at least one imaging mode, where the at least one imaging mode comprises volume imaging, CT imaging, panoramic imaging, or cephalometric imaging.
 3. The apparatus of claim 1, where the dental imaging apparatus is configured to operate in three or more imaging modes, where the three or more imaging modes comprises a combination of volume imaging, CT imaging, panoramic imaging, or cephalometric imaging.
 4. The apparatus of claim 1, where the dental imaging apparatus is configured to operate in two or more imaging modes, where the two or more imaging modes comprises a combination of volume imaging, CT imaging, panoramic imaging, or cephalometric imaging.
 5. The apparatus of claim 4 comprising at least one second digital imaging sensor that is alternately used by the dental imaging apparatus and to provide image data according to received radiation, where the dental imaging apparatus comprises at least an extra-oral dental imaging apparatus or an intra-oral dental imaging apparatus.
 6. The apparatus of claim 4 where the dental imaging apparatus is configured to selectably operate in both of a panoramic imaging mode and a cephalometric imaging mode, where the cephalometric imaging mode is configured to use the first digital imaging sensor, and where the panoramic imaging mode is configured to use the first digital imaging sensor.
 7. The apparatus of claim 6 where the dental imaging apparatus is configured to move the first digital imaging sensor to a first imaging position for use in the cephalometric imaging mode and to a second imaging position for use in the panoramic imaging mode, where the first digital imaging sensor is configured to move between the first imaging position and the second imaging position by operator action, responsive to an operator selection or automatically based on imaging mode.
 8. The apparatus of claim 1 wherein the mount is coupled to a rotational actuator that is energizable to revolve the imaging sensor and source in a scan pattern about the patient's head.
 9. The apparatus of claim 8 further comprising one or more vertical actuators energizable for changing the relative vertical position of the imaging sensor and the radiation source to the patient's head and wherein the computer combines two or more images in the series to form a helical computed tomography image, wherein the one or more vertical actuators translate the mount for changing the relative vertical position of the imaging sensor and the radiation source to the patient's head during imaging.
 10. The imaging apparatus of claim 1 further comprising an alignment apparatus for providing alignment of the patient's head for obtaining the one or more images.
 11. The imaging apparatus of claim 1 further comprising: a memory in communication with the computer for storing the one or more two-dimensional images; and at least one height sensor in communication with the computer.
 12. The imaging apparatus of claim 1 wherein the at least the first energy threshold is selectable.
 13. The imaging apparatus of claim 1 wherein the imaging sensor further provides, for each of the plurality of image pixels, a second digital value obtained from a count of photons of ionizing radiation energy that exceed a second threshold and an upper threshold, where the at least the first digital value is according to the count of received photons that both exceed the first energy threshold and are less than the upper threshold, and where the second digital value is according to the count of received photons that both exceed the second energy threshold and are less than the upper threshold.
 14. An imaging apparatus for obtaining a volume image of at least a portion of a patient's head, the apparatus comprising: a rotatable mount comprising a radiation source and a digital imaging sensor and coupled to a rotational actuator that is energizable to move the imaging sensor and source in a scan pattern about the patient's head; a computer in signal communication with the digital imaging sensor for acquiring a plurality of two-dimensional images at successive positions along the scan pattern; and a patient support structure to provide a spatial relationship to the scan pattern during the scan movement; wherein the imaging sensor provides, for each of a plurality of image pixels, a digital value according to a count of received photons that exceed at least one energy threshold.
 15. The imaging apparatus of claim 14 further comprising one or more vertical actuators energizable for changing the relative vertical position of the imaging sensor and the radiation source to the patient's head during the revolution.
 16. An extra-oral dental imaging apparatus for obtaining an image from a patient, the apparatus comprising: a radiation source; a first digital imaging sensor that provides, for each of a plurality of image pixels, at least a first digital value according to a count of received photons that exceed at least a first energy threshold; a mount that supports the radiation source and the first digital imaging sensor on opposite sides of the patient's head; a computer in signal communication with the digital imaging sensor for acquiring a first two-dimensional image from the first digital imaging sensor; and a second digital imaging sensor that is alternately switched into place by the mount and that provides image data according to received radiation. 